The invention relates to a process for the photo-optical measurement of the absorption behavior of solid, liquid and gaseous media, especially in the photometric analysis of gases by infrared rays, by exposing the medium being analyzed and a standard medium to light beams broken up by a chopper device into a periodical succession of light pulses, and evaluating the measured difference between the beam attenuations in the tested medium and the standard medium in circuitry which is adjustable as regards the radiation intensity of the optical light source and the sensitivity of the light receiver.
The use of intermittent measuring light beams is common in the photo-optical measuring art. The expression, "light modulation," is also used, and the devices used for this purpose are referred to as choppers. The light modulation is performed mainly for the purpose of separating the actual measuring signal from noise signals, the noise signal having a different frequency than the measuring signal. Also, some common detectors, especially gas detectors using micro-flow sensors, are suitable only for the reception of intermittent light radiation.
The invention finds application mainly in the field of gas analysis through the determination of the absorption of infrared radiation. Such processes and apparatus are the state of the art (German "Auslegeschrift" No. 1,296,839 and 1,698,218).
Light choppers or modulators for photometers are known in a great number of versions. In most cases they involve an electric motor on whose shaft there is fastened a chopper wheel provided with apertures. In the known choppers the apertures are of the same size and uniformly distributed over the circumference of the chopper wheel, so that a pulse sequence of constant intensity is formed, assuming that the conditions of measurement and the medium being analyzed are the same (German "Auslegeschrift" No. 1,946,211, FIG. 2).
In photometers using infrared radiators it is common to check and, if necessary, adjust the sensitivity by means of a defined beam attenuation using calibrating gas or a test diaphragm in the measuring channel. This is necessary because, among other reasons, changes of intensity in the radiation of the infrared source and in the sensitivity of the measuring light receiver are virtually unavoidable, so that correction of the measuring apparatus is necessary at more or less regular intervals of time. It is disadvantageous that, in the known apparatus, a zero check or adjustment is first required before making the sensitivity check, the time constant for the indication of the correction being the same as or similar to that of the actual measuring signal. The requirement that the absorption in the medium being analyzed be zero for the zero level adjustment signifies an often unpleasant limitation in apparatus that are in constant use. For this purpose the medium being analyzed has to be removed and the radiation in the analysis medium has to be given a defined attenuation. In this case the reading must be equal to a given calibration value. If it is not, the sensitivity has to be readjusted. In one known apparatus, a pivotable screen has to be swung into the standard channel for the purpose of the adjustment (German Pat. No. 2,042,727). Such measures must generally be performed separately from the measurement operation and consequently they are time-consuming. If the sensitivity varies undetected during operation, the readings will contain an error.